The invention relates to a method and a device for remote sensing of objects. Within trade and industry the interest in non-optical and non-contact sensing of objects have increased lately. In stores and superstores it is interesting to read price labels and similar elements, and within the industry it is desirable to read identification labels in industrial materials flows. A plurality of non-optical and non-contact principles of identification of labels are used at present. Most common is perhaps antitheft labels in shops.
In a commonly used embodiment in trading, labels are used which are provided with a resonant circuit comprising a capacitive and an inductive element. Said resonant circuit can be forced to resonance by applying an electromagnetic signal having a defined energy content at the resonant frequency to said label in an interrogation zone normally provided at the exit of the shop. A detection device detecting signals from said resonant circuit at the resonant frequency produces an alarm if a label provided with an "activated" resonant circuit enters into said interrogation zone. A practical embodiment of a resonant circuit intended for this purpose is disclosed in U.S. Pat. No. 4,578,654. A complete system includes also a device for "deactivating" said label which is done on payment of the merchandise on which the label is attached to.
A problem of all previously known labels used for the purpose of limiting pilferage is a lack of ability of individually identifying each label when a plurality of labels is present simultaneously in said interrogation zone. For the purpose mentioned the problem is not relevant because an alarm should be delivered independently if one or a plurality of labels having an "activated" resonant circuit enters into said interrogation zone and is detected.